A press worm or screw, especially for a filter press, generally comprises a sleeve or the like mounted upon the press shaft and formed with a thread or flight which fits closely into a surrounding housing. The screw is used to generate pressure by the displacement of solid materials, e.g. to press liquids therefrom.
A typical application of a filter press of this type, whose housing is formed with perforations or openings for discharge of the liquid phase, is in the pressing of oils from eatable-oil seeds or the like. Such a press can be used, for example, to press oil from soy beans, cocoa beans and sesame seeds.
It has been proposed heretofore to provide the flight or thread of a worm or screw type press of the aforedescribed type with plates or the like which are composed of a hardened material and are mounted upon the worm body, these plates being releasably connected with the body. Such a system is described, for example, in open German application (Offenlegungsschrift) DT-OS No. 1 927 705.
In addition to this technique it is also known to provide wear resistant or hard-facing coatings upon the surfaces of the flight of the worm most subjected to wear by deposit welding or by flame-spray deposition, plasma deposition or the like. The flight of the worm, along the surfaces coated with such materials, is then subjected to machining to impart the desired finish and dimensions to the worm.
The mounting of plates on the worm body has been found to be extremely expensive construction technique since a large number of precisely dimensioned and shaped parts is required. Furthermore, the formation of hard-facing coatings on the flight of the worm has also been found to be disadvantageous, especially in the case of filter presses for the aforedescribed purposes, because the hard-facing layer is frequently brittle and is sensitive to breakage.
Conventional deposit welding techniques, for example, invariably have required expensive mechanical working or machining of the deposit-carrying surfaces with previous softening-type annealing and subsequent furnace hardening.
When such thermal operations are carried out, the layer is subjected to stress especially if it is not of extremely uniform or constant thickness. As a result of thickness variations, for instance, the weld-deposited layer can be subject to spalling.